For coating car bodies, generally a method of forming a multilayer coating film on an electrocoated coating object by 3-coat-2-bake (3C2B) system has been widely adopted, in which application of an intermediate i curing by baking→application of a water-based base paint→preheating (preliminary heating)→application of a clear paint→curing by baking are successively conducted. In recent years, however, from the viewpoint of energy-saving, 3-coat-1-bake system is attempted, in which the bake-curing step after application of the intermediate paint is omitted, and after applying an electrodeposition paint on a coating object, application of a water-based intermediate paint→preheating (preliminary heating)→application of a water-based base paint→preheating (preliminary heating)→application of a clear paint→curing by baking are successively conducted (see, for example, JP 2002-282773A).
However, in the 3-coat-1-bake system coating, the intermediate paint, base paint and clear paint are applied one on another in uncured state, and layer mixing is apt to occur between adjacent coating films, degrading smoothness or distinctness of image of resulting film in some occasions.
As a countermeasure thereto, JP 2004-275966A discloses a method for forming laminated coating film excelling in finished appearance and chipping resistance, by carrying out the bake-curing step in multi-stages comprising a low-temperature heating stage and high-temperature heating stage under specific temperature and time conditions. However, this coating film-forming method has a problem that sufficient smoothness and distinctness of image are not always obtained when water-based paints are used as the intermediate paint and base paint.
JP 2003-213218A discloses a method for improving the smoothness of a film formed from a thermosetting liquid coating composition, in which, in the occasion of heat curing the coating composition as applied onto a coating object, the storage modulus G′ of the film at a stress of 0.5 Pa and a frequency of 0.1 Hz is controlled to fall within a range of about 0.5-20 Pa, the loss modulus G″ at a stress of 0.5 Pa and a frequency of 0.1 Hz, to fall within a range of about 1.0-20 Pa, and the storage modulus/loss modulus ratio (G′/G″), to fall within a range of about 0.3-1.0, as measured at the temperature at which the thermal fluidity of the film reaches the maximum before the start of the curing reaction. However, the method is subject to a problem that sufficient smoothness and distinctness of image are not always obtained in the coating by above 3-coat-1-bake system, in particular, by the 3-coat-1-bake system using water-based paints as the intermediate paint and base paint.